VOTE-3184 replace legacy % placeholders with @

[rayestrada]

Jira ticket

https://cm-jira.usa.gov/browse/VOTE-3184

Description

Update % placeholders to use @ for consistency. This will require updating the content in the CMS as a post-deploy step.

Deployment and testing

Post-deploy steps

1. lando drush cr

QA/Testing instructions

1. visit http://vote-gov.lndo.site/ and expand the government banner and see the % placeholders
2. edit the block http://vote-gov.lndo.site/admin/content/block/1 and replace %.gov% with @gov, replace %lock_icon% with @lock_icon and %https% with @https then save the block.
3. revisit http://vote-gov.lndo.site/ and expand the government banner and confirm the placeholders are being replaced correctly.

Checklist for the Developer

• A link to the JIRA ticket has been included above.
• No merge conflicts exist with the target branch.
• Automated tests have passed on this PR.
• A reviewer has been designated.
• Deployment and testing steps have been documented above, if applicable.

Checklist for the Peer Reviewers

• The file changes are relevant to the task objective.
• Code is readable and includes appropriate commenting.
• Code standards and best practices are followed.
• QA/Test steps were successfully completed, if applicable.
• Applicable logs are free of errors.